Process and apparatus for drying, cooling, and conveying extrudate

ABSTRACT

Hot, moist extrudate is discharged into an envelope of cocurrently moving gas which is maintained in a flash-drying zone for cooling, drying and uniform movement of the extrudate away from the point of discharge; also, the escape of vapors and fines is effectively controlled by providing a differential pressure across said zone.

United States Patent [72] lnventor [21 1 Appl. No. [22] Filed [45] Patented [73] Assignee [54] PROCESS AND APPARATUS FOR DRYING,

COOLING, AND CONVEYING EXTRUDATE [56] References Cited UNlT ED STATES PATENTS 2,041,142 5/1936 Norvig 34/20 3,222,797 12/1965 Zies 34/17 3,262 2 l 5 7/1966 Davis et a1. 34/17 Primary Examiner-Meyer Perlin Assistant Examiner-- Ronald C. Capossela Attomey-Young and Quigg 8 Claims 1 Drawing ABSTRACT: Hot, moist extrudate is discharged into an en- [52] U.S.Cl 34/17, velope of cocurrently moving gas which is maintained in a 34/79, 34/81 flash-drying zone for cooling, drying and uniform movement [51] lnt.Cl F26b 7/00 of the extrudate away from the point of discharge; also, the [50] Field of Search 34/14, 17, escape of vapors and fines is effectively controlled by provid- 70. 79, 81 ing a differential pressure across said zone.

r l I 7 g l1 PROCESS AND APPARATUS FOR DRYING, COOLING, ANlD CONVEYING EXTRUDATE This invention relates to an improved method and apparatus for cooling and drying extrudable materials and for controlling and confining finely divided particles and volatile materials which are incidental to the process.

One known process for drying materials involves compressing them in an appropriately designed means, such as an extrusion drier, to cause relatively high temperatures and pressured to be formed within the materials and then discharging the compressed materials from the means through an orifice or die where they, thereby, pass from a region of high temperature and pressure to a region of relatively low temperature and pressure. This change in temperature and pressure causes the fluid wetting the material to change rapidly from the liquid state to the vapor state. Accompanying the change in state is a large and rapid increase in fluid volume which virtually explodes the material as the vapor escapes. This change in state, called flashing, occurs within close proximity of the orifice. it is believed that the vapor formed on flashing can be described as a saturated or very slightly superheated gas. The partially dried material formed on flashing is hot, highly porous, spongy, and exhibits a tacky condition. Also, due to its light, porous state, it is susceptible to erratic motion caused by irregular gas currents and differential pressures which are generally present in the flash zone.

Extrudable material, for example polymers, discharged from the die of an extruder-drier as mentioned above is generally only partially dry. The extrudate is ordinarily less than 10 percent by weight wetting fluid, usually on the order of 2 to 3 percent by weight'wetting fluid. It is therefore necessary to further dry the material in order to properly prepare it for further processing. Also, the extrudate, as stated above, is hot and it must be cooled in order to facilitate further processing. It is known to pass the extrudate from the drier into an enclosure containing a cooling and drying medium and a means of transport for the dried material. The cooling and drying medium has been air introduced into the enclosure in random fashion. This method of introduction has caused the extrudate to swirl and blow back against the hot die face where the tacky extrudate sticks, oxidizes, and cakes. The caked material can eventually fall into the dried material, and is a potential source of product contamination. it is therefore important to provided for uniform movement of the extrudate away from the die face as it is discharged therefrom in order to avoid or otherwise reduce any tendency of the extrudate to blow back against the die face.

Another problem encountered subsequent to the extrusion is premature condensation of vaporized wetting fluid. it is necessary to provide conditions to prevent premature condensation of the vaporized wetting fluid. Premature condensation rewets the material and is thus a source of product contamination and manufacturing inefficiency. Two sources of premature condensation include gas which is not warm enough to retain the vaporized wetting fluid manufacturing inefficiency. in the vapor state and surfaces of the drier enclosure which are cool enough to induce nucleation and subsequent condensation.

It has been observed that the tendency of the dried material to erratic movement is magnified by its small size. Thus, very finely divided materiadustIike--has a great tendency to migrate to and contaminate adjoining spaces and, due to its tacky consistency, to stick and build up on inconvenient and unintended locations. It has also been observed that the dried material emits noxious vapors which pollute adjacent workspaces. The dust and vapors, when permitted to escape, create unpleasant working conditions and present a plant safety hazard.

l have discovered that by discharging the extrudate and drying-cooling medium into a flash-drying chamber in cocurrent, parallel flow and in such manner that the extrudate is closely and uniformly surrounded on all sides by the drying-cooling mediumthe tendency of the extrudate to blow back against the hot diehead is eliminated. Thus, the introduction of the medium from positions uniformly surrounding, parallel to, and cocurrent with the flow of the extrudate from the die eliminates differential pressures, eddying and stagnant spots. This method of introduction of the drying-cooling medium is an effective means of transporting the extrudate away from the die face as it is discharged therefrom.

I have also discovered that maintaining the drying-cooling medium at a temperature in excess of the boiling point of the wetting fluid at the operating pressure of the flash-drying chamber prevents premature condensation and provides sufficient sensible heat to adequately warm the surfaces of the chamber to prevent nucleation and condensation.

I have further discovered that the emission of noxious vapors and fine particles to the atmosphere can be controlled by inducing a very slight negative pressure at the point where the dried material being transported by a conveying means exits the flash-drying chamber. Placing an exhaust means at another location attached to the flash-drying chamber will permit the drying-cooling medium, noxious vapors, fines, and outside air, which flows to the above-mentioned induced lowpressure point, to exit through an appropriately provided exhaust and trap chamber.

It is thus an object of my invention to provide a method and an apparatus to transport extrudate away from the die and to substantially reduce or eliminate eddying, stagnant spots and differential pressures which cause the extrudate to blow black against the hot diehead. It is another object of my invention to prevent premature condensation of the vaporized wetting fluid. It is a further object of my invention to provide a novel flash-drying chamber which will confine and control the emission of dust and noxious vapors which are incidental to the drying and cooling process.

Other aspects, objects, and the several advantages of my invention will be apparent from the following disclosure, claims and drawing, which shows a longitudinal section through an apparatus for use inthe present method.

As shown in the drawing, heated air, at a temperature in the range of 2l2248 F. and traveling at the rate of 2,500 3,500 standard cubic feet per minute (29.92 in Hg. 60 F.), is admitted into flash-drying chamber 1 through duct 2, which is annular to extruder-drier 3 and having the outlet thereof adjacent to and in the plane of diehead 4. Extrudate, such as butadiene-styrene copolymer at a temperature in the range of 305345 F. is admitted into flash-drying chamber 1 through the outlets in diehead 4 of extruder-drier 3 at the rate of 6,0009,000 pounds per hour. Ambient air is admitted into flash-drying chamber 1 through opening 5 which also serves as outlet for dried extrudate which is transported on vibrating conveyor 6. Heated air, ambient air, extrudate fines, and vapors exit flash-drying chamber 1 through exhaust hood duct 7. Large extrudate fines impinge on baffle 8 and fall to conveyor 6 for removal through opening 5. Small extrudate fines impinge on baffle 9, fall into trap 10 for removal through hinged door 11. Heated air, ambient air and vapors exit through duct 12.

The direction of flow of heated air in duct 2 is substantially parallel to the flow of extrudate as it exits extruder-drier 3 through diehead 4. in the plane of diehead 4, which plane also contains the outlet of duct 2, heated air flows parallel to and cocurrent with the extrudate flow and uniformly, completely, and closely surrounds the extrudate as it leaves diehead 4. Where diehead 4 is slightly larger in diameter than extruder drier 3, sleeve 13, of the same diameter as diehead 4, can be provided, if desired, in order to eliminate the possibility of swirling in the heated air as it passes from duct 2 over diehead 4.

While flash-drying chamber 1 and duct 2 have been illustrated as being rectangular in shape, it is within the scope of this invention to provide a tubular configuration for either one or both of these portions. Likewise, if desired, straightening vanes can be provided within duct 2 to further promote the formation of the required smooth airflow.

If desired, secondary heated air can be admitted into flashdrying chamber 1 through conduit M having a hinged outlet 15, which prevents any backflow of heated air from flash-drying chamber 1.

Vibrating conveyor 6 is connected to the sidewalls of flashdrying chamber '1 by means of flexible connections. The flexible connections provide a positive seal at the bottom of the flash-drying chamber and also permit vibrating conveyor 6 adequate freedom of movement.

In a further embodiment of the invention a pressure drop in the range of about 0.06-0.l2 pound per square inch is provided across the flash-drying chamber. The pressure drop, which can be maintained by fan I6, is utilized to prevent the escape of fines and vapors through opening 5. Under proper operating conditions the highest pressure in the apparatus exists downstream of the outlet of duct 2 in the immediate vicinity of the plane of diehead 4, the lowest pressure in the apparatus exists in duct 12 just upstream of fan 16, and the pressure within the apparatus in the immediate vicinity of opening is less than ambient pressure. The speed of fan 16 is varied in order to maintain the pressure in the immediate vicinity of opening 5 at a value less than ambient pressure.

In one specific embodiment of my invention 6,000 pounds per hour of butadiene-styrene copolymer prepared by the solution process and having a Mooney Viscosity of 50 ML, is compressed in extruder-drier 3 at 325 P. such that the water content is reduced to 3 percent by weight at the point at which the copolymer is discharged into flash drying chamber I through diehead 4. In the plane of diehead 4, immediately subsequent to extrusion, the extrudate is uniformly contacted and completely surrounded with cocurrent, parallel moving air at 248 F., flowing at 3,000 standard cubic feet per minute which is provided through conduit 2 and introduced into flash drying chamber 1 in the plane of diehead 4- at which points the pressure is approximately 0.25 pound per square inch gauge. The extrudate settles on vibrating conveyor 6 which transports it for discharge from flash-drying chamber 1 through opening 5 at which point the extrudate has a water content of 1 percent by weight and is at a temperature of 225 F. It was observed that operating in this manner resulted in no extrudate blowback against the diehead, no caking, no premature condensation, and control of noxious vapors and fines is effected.

While the process of this invention has been illustrated for drying copolymers of butadiene-styrene it is to be understood that any material capable of extrusion-drying can be successfully subjected to the drying and cooling method which I have discovered. Such materials include those selected from the group consisting of homopolymers of conjugated dienes having from 4--8 carbon atoms per molecule and copolymers of said conjugated dienes with vinyl aromatic hydrocarbons having from 8l2 carbon atoms per molecule, emulsion or solution produced butadiene-styrene copolymers, polybutadienes, butyl rubber, neoprene, polyisoprcne, and the like.

Reasonable variations and modifications, which will be apparent to those skilled in the art, can be made in this invention without departing from the spirit or scope thereof.

I claim:

ll. In a process for the reduction of liquid content in an extrudable material wherein said extrudable material is first discharged from an extruder-drier through a die as an extrudate, flashed in a flash-drying zone and thereafter recovered, the improvement which comprises discharging said extrudate through said die into an envelope of moving gas which closely and uniformly surrounds said extrudate and which is flowing parallel to and cocurrent with the movement of said extrudate as it is discharged from said extruder-drier through said die, said envelope of moving gas being formed by introducing gas into said flash-drying zone through an annular space which uniformly surrounds the periphery of said die.

2. The process of claim 1 wherein said moving gas is maintained at a temperature above the boiling point of said liquid wetting said extrudable material at the operating pressure of the flash-drying zone.

3. The process of claim 1 wherein the pressure of the flashdrying zone in the vicinity of the die is maintained slightly above ambient pressure.

4. The process of claim 3 wherein the pressure of the flashdrying zone at the point of ultimate departure of the resulting dried extrudate is maintained slightly below ambient pressure.

5. The process of claim I wherein said extrudable material is selected from the group consisting of homopolymers of conjugated dienes having from 48 carbon atoms per molecule and copolymers of said conjugated dienes with vinyl aromatic hydrocarbons having from 8-12 carbon atoms per molecule, said liquid is water, and said gas is air in the range of 212 to 248 F.

6. An improved apparatus for drying and cooling an ex trudable material which comprises, in combination, an extruder-drier connected at the die end thereof to a flash-drying chamber, means for the continuous removal of dry extrudate from said chamber, and conduit means for the introduction of gas into said flash-drying chamber, said conduit means being positioned to enclose the die-end portion of said extruderdrier and terminate in the plane of the die face to thereby form an annular space around said die such that said gas in introduced into said flash-drying chamber in a manner parallel to, cocurrent with and uniformly and closely surrounding said extrudate as it exits said die.

7. The apparatus of claim 6 wherein there is incorporated exhaust means located at a point downstream of said die to remove gas and fines from said flash-drying chamber and provided with means in operable association therewith to create a slight negative pressure with respect to ambient pressure within said flash-drying chamber at a point near the discharge end of said dry extrudate removal means.

8. The apparatus of claim 6 wherein there is provided second conduit means located upstream, in, and slightly above the plane of said dry extrudate removal means for the introduction ofa second stream of gas in a manner over, parallel to, and cocurrent with the movement of said dry extrudate along said dry extrudate removal means. 

1. In a process for the reduction of liquid content in an extrudable material wherein said extrudable material is first discharged from an extruder-drier through a die as an extrudate, flashed in a flash-drying zone and thereafter recovered, the improvement which comprises discharging said extrudate through said die into an envelope of moving gas which closely and uniformly surrounds said extrudate and which is flowing parallel to and cocurrent with the movement of said extrudate as it is discharged from said extruder-drier through said die, said envelope of moving gas being formed by introducing gas into said flash-drying zone through an annular space which uniformly surrounds the periphery of said die.
 2. The process of claim 1 wherein said moving gas is maintained at a temperature above the boiling point of said liquid wetting said extrudable material at the operating pressure of the flash-drying zone.
 3. The process of claim 1 wherein the pressure of the flash-drying zone in the vicinity of the die is maintained slightly above ambient pressure.
 4. The process of claim 3 wherein the pressure of the flash-drying zone at the point of ultimate departure of the resulting dried extrudate is maintained slightly below ambient pressure.
 5. The process of claim 1 wherein said extrudable material is selected from the group consisting of homopolymers of conjugated dienes having from 4-8 carbon atoms per molecule and copolymers of said conjugated dienes with vinyl aromatic hydrocarbons having from 8-12 carbon atoms per molecule, said liquid is water, and said gas is air in the range of 212* to 248* F.
 6. An improved apparatus for drying and cooling an extrudable material which comprises, in combination, an extruder-drier connected at the die end thereof to a flash-drying chamber, means for the continuous removal of dry extrudate from said chamber, and conduit means for the introduction of gas into said flash-drying chamber, said conduit means being positioned to enclose the die-end portion of said extruder-drier and terminate in the plane of the die face to thereby form an annular space around said die such that said gas in introduced into said flash-drying chamber in a manner parallel to, cocurrent with and uniformly and closely surrounding said extrudate as it exits said die.
 7. The apparatus of claim 6 wherein there is incorporated exhaust means located at a point downstream of said die to remove gas and fines from said flash-drying chamber and provided with means in operable association therewith to create a slight negative pressure with respect to ambient pressure within said flash-drying chamber at a point near the discharge end of said dry extrudate removal means.
 8. The apparatus of claim 6 wherein there is provided second conduit means located upstream, in, and slightly above the plane of said dry extrudate removal means for the introduction of a second stream of gas in a manner over, parallel to, and cocurrent with the movement of said dry extrudate along said dry extrudate removal means. 